Sequences of digital images, obtained for example by digitizing motion pictures or television signals, commonly are compressed for storage on a computer readable and writeable medium for subsequent authoring and playback of multimedia programs using editing systems such as the Media Composer editing system from Avid Technology, Inc. A variety of compression techniques have been developed, including several standards. In some systems each image, i.e., a field or frame, in the sequence of digital images is compressed separately using still image compression techniques. An example of such a compression technique is known as "JPEG," which is an acronym for 37 Joint Photographic Experts Group." This group developed the ISO 10918-1 JPEG Draft International Standard, CCITT Recommendation T.81.
The draft standard is described in JPEG Still Image Data Compression Standard by William B.
Pennebaker and Joan L. Mitchell, New York: Van Nostrand Reinhold, 1993, and in "The JPEG Still Picture Compression Standard" by Gregory K. Wallace, Communications of the ACM, April 1991, pages 31-44.
Using JPEG and other forms of still image compression, the image is subdivided into blocks of picture elements (pixels). Each block is transformed from its color representation in the spatial domain to a color representation in a frequency domain, for example using a discrete cosine transform. The resulting matrix of frequency coefficients, one coefficient for each frequency, is quantized using a set of quantizers, one quantizer for each frequency, to provide a quantized value for each frequency. Each frequency coefficient is divided by the corresponding quantizer. The set of quantizers typically is referred to as a quantization table or quantization matrix. The quantized values are entropy-encoded. In the JPEG standard, entropy encoding is performed by run length encoding followed by Huffinan encoding. Arithmetic coding also may be used.
Adaptive quantization methods change the quantizers, either within an image on block boundaries, or from one image to the next. Quantizers may be modified either to change the data rate or to change the visual fidelity with which the image is reproduced. An increase in a quantizer value decreases the data rate but also loweres the fidelity of the quantized result. Quantizers may be adapted individually, or the table of quantizers may be scaled uniformly by a scale factor. The JPEG standard does not provide for adapting quantizers within an image. However when JPEG is applied to a sequence of digital images, the quantizers may be adapted from one image to the next. The total amount of data produced by compressing an image is compared to a target data rate, from which an adjustment to the quantization table is determined. The adjusted quantization table is applied to the next image in the sequence of digital images. Such a technique is described in U.S. Pat. Nos. 5,577,190 (Peters) and 5,355,450 (Garmon et al.).